1. Field of the Invention
The present invention relates to a triazole derivative, in particular, a triphenyl triazole derivative having a pyridyl group. Further, an embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and lighting equipment in each of which the triphenyl triazole derivative having a pyridyl group is used.
2. Description of the Related Art
A light-emitting element in which an organic compound is used as a light-emitting substance has a simple structure in which only a light-emitting layer containing an organic compound that is a light-emitting substance is provided between electrodes, and has attracted attention as a next-generation flat panel display element because of its characteristics such as thinness, lightweight, high response speed, and low direct current voltage driving. In addition, a display in which this light-emitting element is used is excellent in contrast and image quality, and has wide viewing angle.
The emission mechanism of a light-emitting element in which an organic compound is used as a light-emitting substance is of a carrier injection type. In other words, when voltage is applied with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make the light-emitting substance excited, and light is emitted when the excited state returns to a ground state. As in the case of photoexcitation, the types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
On the other hand, when a compound which converts a triplet excited state into light emission (hereinafter such a compound is referred to as a “phosphorescent compound”) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency that is 3 times to 4 times as much as that of a fluorescence compound can be achieved. For those reasons, in order to achieve a highly efficient light-emitting element, a light-emitting element in which a phosphorescent compound is used has been actively developed recently (for example, see Patent Document 1).
In the case where a light-emitting layer of a light-emitting element is formed using a phosphorescent compound, the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix formed of another substance for suppression of concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation (T-T annihilation). In the above case, a substance which serves as a matrix is referred to as a host material and a substance, like a phosphorescent compound, which is dispersed in a matrix is referred to as a guest material.
In the case where a phosphorescent compound is used as a guest material, a host material is required to have higher triplet excitation energy (a difference in energy between a ground state and a triplet excited state) than the phosphorescent compound.
[Citation List]
[Patent Document 1]
Japanese Published Patent Application No. 2002-352957
However, as for a light-emitting element in which a phosphorescent compound is used as a light-emitting substance, there is a problem in that current efficiency of a light-emitting element which emits blue light is lower than that of a light-emitting element which emits light ranging from red to green.
A phosphorescent compound which emits blue light has a large energy gap. Therefore, a host material which disperses the phosphorescent compound to form a light-emitting layer and a substance which is used for a carrier-transporting layer in contact with a light-emitting region of the light-emitting layer need to have a larger energy gap.
If a material whose energy gap is not large enough is used as a host material of a light-emitting layer or a material for a layer in contact with a light-emitting region, the energy of excitons moves to the material, which causes decrease in emission efficiency of the light-emitting element.
It is an object of an embodiment of the present invention to provide a novel triazole derivative which has a large energy gap and can be used for an electron-transporting layer of a light-emitting element or as a host material. It is another object thereof to provide a light-emitting element having high emission efficiency. It is still another object thereof to provide a low power consumption light-emitting device, electronic device, and lighting equipment.